An optical communication that is capable of a long distance and bulk transfer is known. Particularly in a long distance communication, lots of optical communication techniques have been put to practical use. In general, a semiconductor laser is used as a light source in a transmitting device of the optical communication. With regard to the semiconductor laser, low electrical resistance is preferable. It is because increase in power consumption and heat generation cause deterioration of device characteristics and device lifetime, and may possibly cause deterioration of modulation rate. Especially with regard to a vertical cavity surface emitting laser (VCSEL), areas of an electrode and an active layer are small and thus the resistance and thermal resistance are high. The heat generated by those resistances has a significant impact, which causes the output and the modulation rate to be limited. For this reason, a technique that can keep the electric resistance low is particularly important with regard to the vertical cavity surface emitting laser. On the other hand, with regard to a semiconductor laser such as a pump laser having a long cavity length, its resistance is comparatively small while its operation current is large. This also leads to increase in heat generation and hence causes output saturation. Therefore, a technique that can further reduce the resistance is desired with regard to the pump laser as well.
To increase an area through which current flows is effective for lowering the resistance. Therefore, lowering of the resistance has been tried by increasing a current confinement width or an active layer stripe width. In the case of the vertical cavity surface emitting laser, however, the increase in the current confinement width generally causes decrease in modulation bandwidth. Moreover, the increase in the current confinement width leads to multi-mode, which is unsuitable for a communication where a single-mode fiber is used. Also, in a case of an edge emitting type laser, the increase in the active layer stripe width causes multi-mode, which is of a problem.
In order to solve the above-mentioned problems, a technique that uses a tunnel junction to invert carriers (electrons-holes) is proposed. More specifically, most of p-type semiconductor with high resistance is replaced by n-type semiconductor due to the carrier inversion, and thereby the resistance is reduced. Such a structure is often employed in the vertical cavity surface emitting laser in the 1.55 micrometer band. However, electric properties of the conventional tunnel junction were not sufficient. That is to say, voltage drop, namely the resistance is high in the conventional tunnel junction, which cancels the effect of the replacement by the n-type semiconductor. Thus, the sufficient lowering of the resistance can not be achieved as a whole. Moreover, in the tunnel junction, carrier tunneling probability becomes lower as the band gap becomes larger. Therefore, the resistance of the tunnel junction section becomes higher as the wavelength of the laser becomes shorter. For this reason, the tunnel junction is scarcely employed in the vertical cavity surface emitting laser in the 1.3 micrometer band, for example.
The followings are publicly known as documents in which a tunnel junction having a hetero-junction interface is described. According to the technique described in Japanese Laid-Open Patent Application JP-P2002-134835A, compound semiconductor whose composition is different between the p-side and the n-side of the junction is used. More specifically, an energy level at the conduction band edge of the n-side semiconductor is lower than an energy level at the conduction band edge of the p-side semiconductor, and an energy level at the valence band edge of the n-side semiconductor is lower than an energy level at the valence band edge of the p-side semiconductor. In other words, a so-called Type-II hetero-junction is used. Also, according to the technique described in Japanese Laid-Open Patent Application JP-P2004-336039A, an InGaP layer is inserted on the n-side of the junction, and concentration of Si as the n-type dopant is increased as compared with a GaAs layer.
In National Publication of the Translated Version of PCT Application JP-P2003-518326, a type-II interband hetero structure diode is described. The diode includes a hetero structure consisting of a first layer of InAs and a second layer of GaSb or InGaSb. The first layer and the second layer are separated by an interface layer made of aluminum antimonide. The interface layer is sufficiently thin and a bias tunneling phenomenon occurs through the interface layer.